5-Fluorouridine is a compound first synthesized in 1959 (U.S. Pat. No. 2,885,396) know to have anti-malignant tumor activity. Despite its superb anti-malignant tumor activity, 5-fluorouridine has not been put to use in practice because of the disadvantage that strong side effects follow. A number of attempts to overcome such disadvantage by converting 5-fluorouridine into different derivatives (see, for example, Japanese Laid-open Patent Applns. Nos. 64280/75, 82079/75, 83378/75 and 52183/76) have been made, but useful compounds have not been obtained yet.
Although the present inventors have previously provided new 5-fluorouridine-5'-O-acyl compounds (Japanese Laid-open Patent Applns. Nos. 77298/81, 91996/82 and 91997/82), even these 5'-O-acyl compounds do not possess satisfactory properties from the practical point of view.